1. Field of the Invention
The invention relates generally to the field of roller cone (“rock”) bits used to drill wellbores through earth formations. More specifically, the invention is related to the structure of cutting elements (“inserts”) used in roller cone bits having a single roller cone.
2. Background Art
Roller cone drill bits are commonly used in the oil and gas industry for drilling wells. FIG. 1 shows one example of a roller cone drill bit used in a conventional drilling system for drilling a well bore in an earth formation. The drilling system includes a drilling rig 10 used to turn a drill string 12 which extends downward into a wellbore 14. Connected to the end of the drill string 12 is a roller cone-type drill bit 20.
As shown in FIG. 2, roller cone bits 20 typically comprise a bit body 22 having an externally threaded connection at one end 24, and at least one roller cone 26 (usually three as shown) attached at the other end of the bit body 22 and able to rotate with respect to the bit body 22. Disposed on each of the cones 26 of the bit 20 are a plurality of cutting elements 28 typically arranged in rows about the surface of the cones 26. The cutting elements 28 can be tungsten carbide inserts, polycrystalline diamond inserts, boron nitride inserts, or milled steel teeth. If the cutting elements 28 are milled steel teeth, they may be coated with a hardfacing material.
When a roller cone bit is used to drill earth formations, the bit may experience abrasive wear. Abrasive wear occurs when hard, sharp formation particles slide against a softer surface of the bit and progressively remove material from the bit body and cutting elements. The severity of the abrasive wear depends upon, among other factors, the size, shape, and hardness of the abrasive particles, the magnitude of the stress imposed by the abrasive particles, and the frequency of contact between the abrasive particles and the bit.
Abrasive wear may be further classified into three categories: low-stress abrasion, high-stress abrasion, and gouging abrasion. Low-stress abrasion occurs when forces acting on the formation are not high enough to crush abrasive particles. Comparatively, high-stress abrasion occurs when forces acting on the formation are sufficient to crush the abrasive particles. Gouging abrasion occurs when even higher forces act on the formation and the abrasive particles dent or gouge the bit body and/or the cutting elements of the bit.
As a practical matter, all three abrasion mechanisms act on the bit body and cutting elements of drill bits. The type of abrasion may vary over different parts of the bit. For example, shoulders of the bit may only experience low-stress abrasion because they primarily contact sides of a wellbore. However, a drive row of cutting elements, which are typically the cutting elements that first contact a formation, may experience both high-stress and gouging abrasion because the cutting elements are exposed to high axial loading.
Drill bit life and efficiency are of great importance because the rate of penetration (ROP) of the bit through earth formations is related to the wear condition of the bit. Accordingly, various methods have been used to provide abrasion protection for drill bits in general, and specifically for roller cones and cutting elements. For example, roller cones, cutting elements, and other bit surfaces have been coated with hardfacing material to provide more abrasion resistant surfaces. Further, specialized cutting element insert materials have been developed to optimize longevity of the cutting elements. While these methods of protection have met with some success, drill bits still experience wear.
As a bit wears, its cutting profile can change. One notable effect of the change in cutting profile is that the bit drills a smaller diameter hole than when new. Changes in the cutting profile and in gage diameter act to reduce the effectiveness and useful life of the bit. Other wear-related effects that are less visible also have a dramatic impact on drill bit performance. For example, as individual cutting elements experience different types of abrasive wear, they may wear at different rates. As a result, a load distribution between roller cones and between cutting elements may change over the life of the bit. The changes may be undesirable if, for example, a specific roller cone or specific rows of cutting elements are exposed to a majority of axial loading. This may cause further uneven wear and may perpetuate a cycle of uneven wear and premature bit failure.
One particular type of roller cone drill bit that merits special consideration with respect to bit wear includes only one leg, bearing journal, and roller cone rotatably attached thereto. With respect to this type of bit, generally known as “single roller cone bits,” they are useful when drilling small diameter wellbores (e.g., less than about 4 to 6 inches [10 to 15 cm]). With single cone roller bits, the drill diameter of the single roller cone is substantially concentric with an axis of rotation of the drill bit. Single roller cone bits may use a significantly larger radial bearing for the same bit diameter as a comparable three roller cone bit. The larger radial bearing enables the use of higher bit loads and may enable increases in the rate of penetration of the drill bit as a result. The single roller cone typically has a hemispherical shape and drills out a “bowl” shaped bottom hole geometry. The single roller cone drill bit efficiently drills the portion of the wellbore proximate the center of the well because the structure of the single cone bit places a large portion of the cutting structure in moving contact with the formation at the center of the hole.
One of the limitations of single cone roller bits is that the cutting elements used in the cone body tend to wear over time due to the shearing action, especially in view of the fact that selected cutting elements are generally in substantially constant contact with the formation being drilled. This is an important consideration in bit design because an important performance aspect of any drill bit is its ability to drill a wellbore having the full nominal diameter of the drill bit from the time the bit is first used to the time the cutting elements are worn to the point that the bit must be replaced. In the case of a single roller cone bit, essentially, all but a few centrally positioned cutting elements on the single roller cone eventually engage the wellbore wall at the gage diameter. The cutting elements on a single cone roller bit go through large changes in their direction of motion, typically anywhere from 100 to 360 degrees. Such changes require special consideration in design. The cutting elements on a single cone bit undergo as much as an order of magnitude more shear than do the cutting elements on a conventional two or three cone bit. Such amounts of shear become apparent when looking at the bottom hole patterns of each type of bit.
A single cone bit creates multiple grooves laid out in hemispherically-projected hypotrochoids. A two or three cone bit, in contrast, generates a series of individual craters or indentations. Shearing rock to fail it will typically cause more wear on a cutting element than indenting a cutting element to compressively fail rock. Therefore, the cutting elements on a single cone roller bit wear faster than the cutting elements on a two or three cone bit. As the cutting elements on a single cone bit wear, therefore, the drilled hole diameter reduces correspondingly.
As the cutting structure wears, the drilled diameter of the wellbore may be substantially reduced because of worn or broken cutting elements. The reduction in wellbore diameter can be an intolerable condition and may require reaming with subsequent bits or the use of reamers or other devices designed to enlarge the wellbore diameter. Moreover, the reduced wellbore diameter will decrease the flow area available for the proper circulation of drilling fluids and bit cuttings. The use of bits, reamers, or other devices to ream the wellbore can incur substantial cost if the bottom hole assembly must be tripped in and out of the hole several times to complete the procedure.
What is needed, however, is a cutting element structure for a single cone roller bit having preferential wear characteristics and that is “self-sharpening” in order to increase penetration efficiency and extend overall bit life.